Some Pointers On Covering And Doping By Michael V. Sharp, EAA 5602 ike many others, I found it a L bit difficult to decide what to make on conflicting information on the subject of covering and doping. On the assumption that those who specialize in the manufacture and sale of such materials know the most about it, I wrote to two firms for information—Cooper Industries, Inc. of Elk Grove Village, 111., and Airtex Products, Morrisville, Pa. Both were very cooperative and I thought a digest of this information would help other EAA members. So here goes! Only two fabrics are approved by FAA for use on certified airplanes, Grade A Cotton and Irish Linen. By inference, these should also be used on amateur-built planes but it should be noted that Civil Aeronautics Manual 04, "Airplane Airworthiness", contains the following remark: "Fabric for aircraft having wing loadings and speeds substantially lower than noted, or fabric that does not meet these specifications in all respects but has been proven by extensive satisfactory service experience to be suitable for aircraft use will be subject to special consideration". We can experiment. The name of the manufacturer and the specification CAA-TSO-C-15 must be stamped on the selvedge of cotton cloth to show that it is Grade A aircraft material, while a statement to the effect that the material meets British Specification DTD-540 must accompany all purchases of Irish linen. It is hard to say which material is "best" because each has its own distinctive properties. There is a difference of opinion as to which is the most susceptible to mildew. One vendor says of linen that "It is considered resistant to mildew" while another states "Despite information to the contrary, linen is more susceptible to mildew than cotton". This being the case, and the procedure being simple enough anyway, it seems a good idea to use fungicidal treatment on both cotton and linen. Linen has a higher tensile strength than cotton—but of course cotton does have adequate strength. It is generally conceded that linen will outlast cotton. Cotton is easy to work with and takes dope more readily; during the weaving process linen is lubricated with small quantities of tallow, which naturally affects dope penetration. This problem is 10

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not insuperable though, because thinning of the first coat of dope (especially butyrate) about 50% aids penetration adequately. Pricewise, the picture seems irregular; sometimes one pays about the same, sometimes a little more or a little less for one or the other. Mildew proofing can be done by adding a special preparation to plain dope, or by means of special coatings which might be described as serving the same purpose as a primer on wood or metal. Such materials commonly have tinting, such as green, which serves as a telltale. If the color can be seen showing on the inner surface of the fabric it indicates thorough coverage. Fabric is attached to the framework in the form of tailored, sewn-up covers or merely by doping it to the members which form the edges of the structure. The latter method predominates in the lightplane field because it is quicker and easier, and is adequately reliable on such machines. Government publications and airplane maintenance textbooks contain full details on approved stitching, lacing and taping materials and methods and should be consulted. Where a number of strips of cloth must be sewn together edge-to-edge, such as to make a wing cover, the seams must comply with regulations.

Special sewing machines are made which can fold and double-stitch the seams automatically and as they do the job so much better and faster than household sewing machines, a number of aircraft supply firms such as those previously mentioned offer ready-sewn envelopes. Correct fabric tension is important because it is closely connected to dope penetration, strength and smoothness. One expert says that fabric should be snug but not stretched. On large and long parts, such as fuselages, it is easy to pull cloth tighter in one direction than in the other. Then when it is doped it will pull up more one way than the other and wrinkles may result. Too-tight fabric has the interstices between the weave opened up, more dope film goes into them, and when it dries the tightening effect is that much greater. Lightweight framework can easily be pulled out of shape as a result. The "intermediate" grade fabric formerly used on light airplanes had more threads per inch than Grade A, consequently the interstices were smaller and distortion of light frames was less likely. However, as thread size was smaller tear resistance was less and today Grade A is recommended in its place. It is common practice to apply water to fabric to draw it snug and

This is how they do it in a mass production factory. This shot, taken some time ago in the Piper factory, shows a conveyor system used to hold wings and fuselages for convenient spraying and sandpapering.

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pull out any wrinkles which may inadvertently have been obtained during fitting and attachment. Textile mills rinse Grade A fabric very thoroughly in clean water during the finishing process to remove all impurities. Obviously if the fabric is wetted with hard or otherwise impure water, the effect is to cancel the value of such rinsing. The danger is that salts and minerals left in the cloth will absorb moisture in wet weather and lead to early rotting. If in doubt about local tap water, use distilled water and apply it with something you know will not cause contamination. If wetting is done, fabric should dry for at least 24 hours before doping. Its surface may feel dry a few hours after wetting, but there is also moisture in the cotton fibers which will take longer to evaporate. If the atmosphere is very damp or very dry it is best to postpone doping until conditions of moderate humidity have returned; abut 40% to 70% at 65 to 85 degrees F. is suggested. If the air is abnormally wet or dry during doping, the fabric will naturally return to normal moisture content with the return of normal humidity and this can harm the dope-to-fabric bond. Also, since dope dries by evaporation it becomes cooler than the surrounding air while drying. If the air is very humid this can cause water to condense on the dope surface and cause "blushing". This is a milky appearance and is readily identified when seen. It weakens clear dope and spoils the gloss of pigmented dope and is to be avoided. When work must be rushed to completion in damp •weather, "retarder" can be added to dope to slow down the drying and, by reducing chilling, also reduce the possibility of blushing. In very hot, dry weather dope may dry so rapidly as to give considerable trouble in brushing out smoothly and uniformly. The objective when painting is to spread paint as thinly and uniformly over a surface as possible, but when doping the objective is to saturate the surface. Fill the brush with dope, flow it on, and brush it out just enough to avoid puddles and runs. Avoid putting so much dope on that it drips through inside the fabric, and do not continue brushing once it has been adequately spread, for its quick drying will cause "roping", or pulling on the brush, and result in irregular deposit of dope. Dope normally dries to the touch within an hour but it is wise to wait several hours before sandpapering. When dope is insufficiently dry it

will gum the sandpaper. If material comes off in the form of dust instead of "rolls", the dope is dry enough to take the next coat. However, it is not wise to let more than 24 hours elapse between coats because we want preceding coats to be soft enough to ensure a good bond with later ones. If it can't be helped, several days can pass between coats but it should be realized that dope eventually becomes so dry and hard that further coats will strike into it unevenly, causing a cracked surface. This is why "rejuvenator" is used before refinishing an old covering job; it softens the old dope so that fresh dope will go on without cracking. Some mechanics wet-sand old surfaces before spraying rejuvenator to clean off dirt and dead surface pigment, and the resulting microscopic scratches let rejuvenator strike more deeply into some areas than others, with an alligatored or cracked finish the inevitable result. Do not apply dope when the preceding coat is still drying, for this traps some of the solvent. Several coats applied too close together result in a skin with a high solvent content; very slow or very poor tautening is the result and no time is saved. When the first coat dries, fabric is usually slack and puckered. The threads of the fabric have absorbed most of the dope and interstices between threads are not filled enough to cause tightening. While some mechanics apply pinked tape over ribs and edges at this point so that both fabric and tape will have the same number of coats, it is usuallybest to apply tape, grommets and inspection rings after the second coat. A smoother job results from laying these materials onto a tighter fabric, and adhesion is better. If ribstitching knots project above ribs, push them down flush with the ribs with your thumb to avoid bumps in the pinked tape. Brush dope onto the surface copiously, lay the tape, and brush more dope on the tape. Push down and work well with the brush and fingertips so that there are no wrinkles or bubbles under the tape. Do not put so much dope around inspection rings that puddles form. These areas will become brittle and the stretching and drumming of fabric in flight will cause cracks in the dope around the circumference of the rings. Some celluloid rings have bevelled outer edges for the purpose of blending into the dope and fabric with minimum discontinuity and least tendency to crack.

When dope has dried on the pinked tape it is common to find the edges rather rough. There may be fuzz and tiny bubbles on much of the fabric surface. Therefore a light sandpapering is advised. Use fine paper and just "wipe" the surface, don't rub it. Be very careful over ribs and near edges not to cut through the dope film or, worse, the fabric threads themselves. The more coats of dope there are, the less is the likelihood of damaging the fabric. This is one of the many fine points which makes a good mechanic! Usually from four to six coats of clear dope are applied. It depends on the job at hand. It would be safer to put only four on a very lightly built plane to avoid excess tension on the framework. If perhaps four coats did not seem to pull the fabric up enough, five could be used. A deluxe job would be helped by the extra filling of six coats. The "solids content" of dope varies and some batches may fill better than other batches or brands. When little of the weave texture shows through the dope skin it is assumed that the job is satisfactory. After the second coat and tapes, grommets, etc. have been installed, it is possible to spray clear dope coats but by no means is it essential. High grade paintbrushes of good width and with long, flexible, well-set bristles do a fast, smooth job. Because dope begins to dry at once and quickly reaches the roping stage, it is practically impossible to get good results trying to brush pigmented dope. After the last coat of clear dope, then, it is essential to change to a spray gun. If one tries to brush pigmented dope over a preceding coat of such dope, the new coat is quite apt to cut into the former and the result is a mess. Because airplane dope is supposed to have some flexibility it does not have as much pigment as common paint and here again it is harder to get thorough, uniform coverage with a brush. Low-cost home workshop sprayguns are of the suctionpot type but production guns feature pressure feed to the nozzle. This means that the latter throws a lot more dope onto a surface with every stroke and while simple guns can certainly do a good job, it may take an extra coat or two to get equivalent results. Most mechanics much prefer cross spraying, in which the gun is run one way and then the other for each coat. This greatly reduces the possibility of getting a streaky finish due to irregular application. The Continued on next page SPORT AVIATION

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POINTERS ON COVERING . . .

Continued from preceding page more you have to thin out dope to get it to spray, the less will each coat cover. Two or more coats of aluminum dope go over clear dope. The purpose is partly to provide a good

sandpapering surface and partly to block sun rays from reaching the clear dope—sunlight causes very rapid deterioration of clear dope. The ideal condition is to have enough aluminum so that absolutely no light can be seen through the fabric from the inside of the wing or fuselage, but this is a rather hard goal to obtain. About three coats when using a pressure pot gun and four, five or six when using a suction feed gun are the normal limit, because we have to remember not to get too much tension and weight. Pre-mixed aluminum dope is marketed but many prefer to make their own by stirring fine-grade aluminum powder into clear dope. The proportions are critical. If too much powder is used you get a "dry" coat or a weakly-bonded one, and color coats are apt to peel off. If too little is used, coverage will be too poor to give the clear dope adequate protection. Use four ounces by weight to each gallon of unthinned dope. Mix the powder into a paste with a small quantity of dope, breaking up all lumps and thoroughly saturating all flakes with dope, before adding to the main container. After the first aluminum coat is dry and tight, sand it thoroughly with No. 280 or linen waterproof sandpaper. The objective of sanding is to deliberately cut the outer surface off the dope to produce something that is almost glass-smooth. There is thus

a lot of dust, and dry sanding can be an agony of floating dust, clogged paper and scratchy surfaces. Wet sanding is far pleasanter and more effective, the water acting as a lubricant. Fold a sheet of paper in half the short way and tear it in two. The resulting pieces can be given two folds, which produces three palmsized panels. When folded this way a coated side presses against each back side and the annoying habit of two back sides slipping against one another is eliminated. Because friction is present there is some danger of static electricity building up and sparks may be produced. It is wise therefore to put a ground wire onto the structure especially if it is of metal. When sanding has been finished the whole surface is covered with "mud", a mixture of dust and water. Use a windowcleaner's rubber blade to wipe it off before it dries. Follow by wiping with clean cloth dampened with water. The fussier you are about cleaning the better will be the end result. After the last aluminum coat has been sanded study the surface closely for small pits where little areas of dope that were lower than the surrounding surface did not get sanded. If a deluxe job is the aim, these little pits should be filled with a few extra spots of dope. Also look at all ribs and edges to be sure protective aluminum dope has not been cut through. Nitrate dope is a better adhesive than butyrate dope and makes the job of applying and taping fabric easier. It also tightens fabric better. However it is very inflammable and its weathering qualities, while good, are notably inferior to butyrate. It is supposed to be feasible to put butyrate dope finishing coats over nitrate

dope undercoats. The cost is only

moderately greater and much improved weather resistance can be obtained. Nitrate dope should not be put over butyrate dope because cracking is likely. Butyrate costs about $1.50 to $2.50 more than nitrate and on a small plane this extra cost will be overbalanced by the improved durability. Acetate dope is no longer used as it has been superseded by butyrate, to which it is related. It was not as good as butyrate. For spraying with a suction gun, dope should be thinned about 50%; with a pressure pot gun 25% reduction is usually adequate. When dope is too thick it spatters onto the surface in globules and gives a rough "orange peel" finish. When it is too thin it not only covers poorly but may develop runs. Air pressure also influences the amount of thinning, for low pressure from a small compressor will not atomize dope as forcibly as will high pressure from a big, husky compressor. On the last finish coat some mechanics like to mix equal quantities

of clear dope, colored dope and re-

tarder. This gives excellent flow-out and produces the best possible gloss on a job which is not to be rubbed and polished. It may have less weather resistance because of the clear dope being next to the sun and weather. Final coats should be carefully protected from dust while drying if

a smooth finish is desired. For more information on covering and doping, see Civil Aeronautics Manual No. 18. Most producers of fabrics and dopes also have literature, some of it quite extensive. You can get such material from your source of aircraft supplies.

MORE ABOUT MUSLIN Since the adjacent article on aircraft fabrics was written, more information on muslin has been received from The American Cotton Manufacturers Institute, Inc., Box 151, Clemson, S.C. A number of substances are used for sizing and include starches such as potato, tapioca, corn, rice and wheat. Corn starch is most common. Also used for sizing are additives such as gums, dextrines and pectins and tallow. All these sizing materials make the cloth rather prone to mildewing and will affect the penetration and adhesion of paint in varying degree. Sizing can be removed by washing the cloth in solutions of acids and enzymes. A special enzyme solution is EXCISE, made by 12

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Pabst Brewing Company, Merchandise Mart, Chicago 54, 111. It may be found that attempts at washing out sizing will leave the muslin with an undesirable wrinkled surface. A number of standard types of muslins are manufactured; a few listed in a trade publication are "wide print cloths", "tobacco cloths" and "sheeting". The aircraft builder wishing to investigate such cottons will have to do so with the realization that he must make his own way in a field which is both extensive and obscure. It might be added here that when the U. S. Government began experimenting with cotton aircraft fabrics during World War One, tests showed

that air pressures on the leading edges of wings were often as much as ten times greater than farther aft. This is the primary reason why we use plywood or metal nosing on fabric covered wings. A small plane such as a Pietenpol or Hannaford Bee with no leading edge reinforcing might fly well and safely for years with Grade A or linen aircraft fabric covering, but an attempt to substitute muslin might well lead to disaster. Remember, pressures on fuselage sides and tail surfaces may be much less than on a wing leading edge. To be on the safe side, use real aircraft cloth, but if you must experiment, do so with plenty of caution and forethought.